In the conversion of solar energy to electricity by a semiconductor photovoltaic cell, incident photons free bound electrons, allowing the electrons to move across the photovoltaic cell. In this process, a photon having energy less than the photovoltaic material's band gap is not absorbed, while a photon having energy greater than the photovoltaic material's band gap only contributes the band gap energy to the electrical Output, and excess energy is lost as heat. Thus, a given photovoltaic cell operates most efficiently when exposed to a narrow spectrum of light whose energy lies just above the photovoltaic material's band gap.
To achieve higher solar energy conversion efficiency than can be obtained with a single photovoltaic material, a number of techniques have been developed to split the broad solar spectrum into narrow components and direct those components to appropriate photovoltaic cells.
In U.S. Pat. No. 2,949,498 to Jackson (1960), a solar energy converter is disclosed that splits the solar spectrum by stacking photovoltaic cells. A high band gap photovoltaic cell is placed in front of one or more photovoltaic cells having successively lower band gaps. High energy photons are absorbed by the first cell and lower energy photons are absorbed by the following cell. This method is disadvantageous in that the leading cells must be made transparent to the radiation intended for the following cells.
Borden et al., Proceedings of the Fifteenth IEEE Photovoltaic Specialists Conference, pp. 311-316 (1981), describes a design in which light is incident upon a dichroic filter that transmits high energy photons to a high band gap photovoltaic cell and reflects low energy photons to a low band gap cell. This method is disadvantageous in that a single dichroic filter yields only two spectral components, and an additional dichroic filter is needed for each additional desired spectral component.
Ludman et al., Proceedings of the Twenty-fourth IEEE Photovoltaic Specialists Conference, pp. 1208-1211 (1994), describes a design in which the spectrum is split by diffraction, and different photovoltaic cells are arranged to capture light of different wavelengths. A hologram serves as the diffraction grating and also concentrates the sunlight. This method is disadvantageous in that it is difficult to economically create durable diffraction gratings having high optical efficiency over a wide portion of the solar spectrum.
While refractive dispersion is a well known means of separating light into its spectral components, it is not trivial to create a refractive optical arrangement that is suitable for solar energy conversion. For example, refractive dispersion designs using only a single array of prisms or a concentrator with a single dispersing prism at or near its focus do not simultaneously provide adequate dispersion and concentration. U.S. Pat. No. 4,021,267 to Dettling discloses a spectrum splitting arrangement comprising concentrating, collimating, and refractive dispersing means. This method is disadvantageous in that the collimating optical element introduces additional transmission losses and alignment difficulties.
Notwithstanding the known problems and attempts to solve these problems, the art has not adequately responded to date with the introduction of a solar energy conversion system which improves efficiency by splitting the solar energy spectrum.